Electronic overload detecting device

Abstract

A device for detecting a persisting electrical overload in an electrical system includes means for monitoring an electrical parameter in the electrical system and for generating an electrical signal whose magnitude represents the instantaneous absolute value of the parameter, reference means which generates an output signal of predetermined value and which develops, in cooperation with the monitoring means, an electrical signal whose magnitude represents the amount by which the magnitude of the absolute value signal exceeds the predetermined value, integrator means to integrate the aforementioned electrical signal and to generate an output whose magnitude includes a component representative of the product of the electrical signal and a time-increasing value that represents the duration that an overload has persisted; and comparator means to continuously compare the magnitude of the integrator output signal with a second preselected value and which generates an indicative output whenever the integrator output signal exceeds the second preselected value. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to an electronic device for detecting overload conditions in electrical systems. 2. State of the Art Currents that exceed the design level of an electrical system (i.e., overloads) may damage the system or its power supply by causing overheating, insulation breakdown, and so forth. Accordingly, it is desirable to have a device which detects an overload condition and which interrupts the current in an electrical system before damage occurs. A well known type of overload detecting device merely compares the current or voltage in a system with a predetermined standard and commands shutdown whenever the standard is exceeded, even momentarily. Those devices do not accomodate the fact that most systems can withstand momentary or nonpersisting electrical overloads without damage. A device recently suggested in U.S. Pat. No. 3,703,679 permits direct current (DC) overloads to persist for a predetermined time period before interrupting the power supply. OBJECTS OF THE INVENTION An object of the present invention is to provide an overload detector which permits electrical overloads to persist for a time dependent upon the magnitude of the overload. Yet another object is to provide an adjustable overload detector usable in alternating current (AC) electrical systems. An advantage of the overload detector of this invention is that its sensitivity and time response characteristics are readily adjustable by manual or automatic means.

We claim: 
     
       1. A device for determining the occurrence of an electrical overload in an electrical system comprising: a. means for monitoring an electrical parameter in an electrical system and for generating an output signal representative of the instantaneous absolute value of the monitored electrical parameter;   b. threshold control means for developing an electrical output signal of predetermined time-invariant value;   c. adder means connected to receive the output signals from said monitoring means and from said threshold control means and to receive a feedback signal, said adder means being operative to generate an output signal representative of the summation of said three received signals;   d. integrator means connected to receive said summation signal from said adder means and to generate an output signal representative of the integral of said summation signal;   e. feedback means coupled between the output of said integrator means and the input to said adder means for feeding said integrated signal back to said adder means with a predetermined gain; and   f. comparator means connected to receive the integrated signal from said integrator means and operative to generate an indicative overload signal whenever the magnitude of said integrated signal exceeds a second predetermined value.   
     
     
       2. A device for determining the occurrence of an electrical overload in an electrical system comprising: a. means for monitoring an electrical parameter in an electrical system and for generating an output signal representative of the instantaneous absolute value of the monitored electrical parameter;   b. threshold control means for developing an electrical output signal of predetermined value;   c. adder means connected to receive the output signals from said monitoring means and from said threshold control means and to receive a feedback signal, said adder means being operative to generate an output signal representative of the summation of said three received signals;   d. first integrator means connected to receive said summation signal from said adder means and to generate an output signal representative of the integral of said summation signal;   i. feedback means coupled between the output of said integrator means and the input to said adder means for feeding said integrated signal back to said adder means with a predetermined gain;   f. second integrator means connected to receive said integrated signal from said first integrator means and to generate an output signal representative of the integral of said integrated signal; and   g. comparator means connected to receive the twice integrated signal from said second integrator means and operative to generate an indicative overload signal whenever the magnitude of said twice integrated signal exceeds a second predetermined value.   
     
     
       3. A device according to claim 2 wherein said predetermined gain of said feeback means is adjustable. 
     
     
       4. A device according to claim 2 wherein said monitoring means comprises a sensor means responsive to the instantaneous value of said monitored parameter and operative to generate therefrom a proportional output signal, and rectifying means connected to receive said proportional output signal and to generate therefrom a rectified signal which is said absolute value signal. 
     
     
       5. A device according to claim 2 wherein said comparator means generates a second output signal when said overload signal is less than said second predetermined value; and first limit circuit means are coupled between said first integrator means and said comparator means to clamp the output of said first integrator to substantially zero whenever said second output signals are received from said comparator means. 
     
     
       6. A device according to claim 4 wherein said rectifying means includes: a. a first detector responsive to AC signals in the monitored system and operative to pass only half cycles of one polarity of said AC signals; and   b. a second detector responsive also the AC signals in the monitored system and operative to pass and invert only half cycles of the other polarity of said AC signals.   
     
     
       7. A device according to claim 5 wherein said comparator means includes: a. an operational amplifier connected to receive said twice integrated signal;   b. a first pair of diodes coupled between the input and the output of said amplifier and joined together anode-to-anode;   c. a second pair of diodes coupled between the input and the output of said amplifier and joined together cathode-to-cathode;   d. means for biasing the anode-to-anode devices to a selected positive value to establish said second predetermined value; and   e. means for biasing the cathode-to-cathode devices to a selected negative value.   
     
     
       8. A device according to claim 2 wherein said predetermined value developed by said threshold control means is adjustable.